1.Field of the Invention
The present invention relates to an optical sheet having an excellent light collecting (condensing) function and an excellent scratch resistance. In addition, the present invention relates to a surface light source device and a transmission-type display device, having such an excellent optical sheet.
2.Description of Related Art
In general, a surface light source device used for a transmission-type display device includes a light source, and a plurality of optical sheets (optical films) that can serve to change a travel direction of light emitted from the light source. For example, as disclosed in FIG. 1 of JP2008-70456A, included in the plurality of optical sheets are a light diffusing sheet that can serve to diffuse the light emitted from the light source, thus shading an image of the light source (rendering the light source image inconspicuous or substantially invisible), and a light collecting sheet (light condensing sheet) that has a function for collecting (condensing) the light travel direction in a front direction, thereby enhancing the front brightness (light collecting function or light condensing function).
As the light collecting sheets, the optical sheets, including unit shaped elements (unit optical elements) extending along a line and arranged orthogonally to the longitudinal direction of each element (this arrangement manner is a so-called “linear arrangement”), are widely used. Typically, each of the unit shaped elements has a ridgeline portion (vertex extending along a straight line) in the longitudinal direction thereof, and has a shape corresponding to a part of a triangular, elliptical or circular shape in a cross section orthogonal to the longitudinal direction.
When the optical sheet including the unit shaped elements extending along a line is incorporated in a transmission-type display device, the optical sheet is superposed on another member such as another optical sheet or the like. In addition, before being incorporated into the transmission-type display device, the optical sheet is sometimes handled (e.g., transported) as a member in the form of multiple overlapped sheets which have been cut into a final shape, or as a member in the form of a wound web-like sheet which is not yet cut into the final shape. Under these conditions, each of the unit shaped elements of the optical sheet is locally in contact with another member, at a narrow area of the linearly extending ridgeline portion. Thus, when there generate slight vibrations or external forces while the display device and the optical sheet are handled, stress may concentrate on the ridgeline portion, resulting in a deformation of the unit shaped element.
In particular, when the optical sheets in their own are transported or when a display device including an optical sheet and another member is transported, there is a possibility that the adjacent optical sheets (or the optical sheet and the other member adjacent thereto) are repeatedly displaced relative to each other, while being pressed on each other. As a result, the surface of the optical sheet on which the unit shaped elements are formed may have visible scratches. Such disadvantage is remarkably likely to occur in the case of unit shaped elements (unit prisms) having a triangular cross-sectional shape, which can exert an excellent light collecting function.
In the optical sheet disclosed by JP2008-70456A, by partially projecting an inclined surface of each of the unit shaped elements having a triangle columnar prism shape, a hemispherical curved lens is disposed on the inclined surface. In particular, FIGS. 4 and 9(c) of JP2008-70456A disclose that a hemispherical curved surface 12G is formed so as to cover the vertex of the triangular cross section. Due to the provision of such a curved surface 12G, the vertex of each of the unit shaped elements having the triangular cross-sectional shape, which tends to be deformed, can be prevented from being directly in contact with another member adjacent to the optical sheet. Thus, a contact resistance of the optical sheet can be possibly enhanced.
According to the disclosure of JP2008-70456A, the curved surface can provide a light diffusing function to the optical sheet. In view of the paragraphs 0036, 0059 and 0060 of JP2008-70456A, there is described only the fact that the optical function (mainly the light collecting function) of the part, which has the triangular cross-sectional shape, of the unit shaped element (prism body), and the optical function (mainly the light diffusing function) of the other part, which is provided with the curved surface, of the unit shaped element, are exhibited independently of each other. That is to say, from the technique disclosed in JP2008-70456A, it cannot be expected to obtain some combined effect of the optical function of the part having the triangular cross-sectional shape and the optical function of the part provided with the curved surface.
Further, in JP2008-70456A, the projected curved surfaces are arranged on the unit shaped elements having a triangle columnar shape, along a direction in which the unit shaped elements extend. Thus, at positions of the projected curved surfaces, the light collecting function of the inclined surfaces of the triangle column is inhibited. In addition, since a part of the hemispherical surface is cut by the triangle column, the light diffusing function of the hemispherical surface is impaired. That is to say, although the optical sheet disclosed in JP2008-70456A has the light collecting function and the light diffusing function, the light collecting function is inferior to that of a perfect triangle columnar prism linear array, as well as the light diffusing function is inferior to that of a so-called fly-eye lens in which only hemispherical curved surfaces are arranged. Accordingly, depending on the weighting balance between the light condensing function and the light diffusing function, the rate of the area that is occupied by such a projected curved surface relative to the whole surface of the unit shaped elements will be controlled. Namely, from the disclosure of JP2008-70456A, it is understood that the provision of the projected curved surface deteriorates the light collecting function of the unit shaped element (prism body itself) having a triangle cross-sectional shape, which causes lowering of the front brightness. Therefore, although the scratch resistance of the optical sheet of JP2008-70456A might be enhanced, such enhancement can be obtained at the sacrifice of the light collecting function.